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KEY POINTS

Bones of the cranial vault are absent but facial structures and skull base are preserved. Residual brain tissue is present and floats free in amniotic fluid.

Likely to be the first trimester precursor to anencephaly. Now called fetal acrania–anencephaly sequence.

Incidence is 3 per 10,000 second trimester pregnancies.

Sonographic findings in the first trimester include: absent calcification of the cranial bones, lateral widening of the cerebral hemispheres (the “Mickey Mouse” sign), and echogenic amniotic fluid. Second trimester findings include free-floating disorganized brain tissue with preservation of the face.

Often associated with omphalocele, amniotic band syndrome, limb–body wall complex, and pentalogy of Cantrell.

CONDITION

Exencephaly is a rare fetal anomaly that is incompatible with extrauterine life. In exencephaly, the bones of the cranial vault are absent (acrania), but the facial structures and the base of the skull are preserved (Casellas et al., 1993). The terms exencephaly and acrania are used interchangeably in this chapter. Exencephaly is a precursor to anencephaly (the so-called fetal acrania-anencephaly sequence); it differs from anencephaly in that residual brain tissue is present and floating free in the amniotic fluid.

Exencephaly is frequently noted in animal teratogen studies. Human exencephaly appears to be confined to early gestation. Only rare reports exist of a third trimester diagnosis of an exencephalic fetus (Wilkins-Haug and Freedman, 1991). Anencephaly, however, is more common in humans than in animals. The greater prevalence of anencephaly in humans is attributed to a longer gestational period, which presents the opportunity for destruction of the free-floating brain matter.

Exencephaly is due to the failure of the anterior neuropore to close during the 4th week of embryonic development. The underlying defect is due to a failure in mesenchymal migration (Stagiannis et al., 1995). In pathologic studies, the exencephalic brain is noted to be covered by a highly vascular epithelial layer. In exencephaly, two relatively equivalent cerebral hemispheric remnants are present within a reddish mass of disorganized tissues, remnants of deep cerebral neural elements, blood vessels, fibrous tissues, and fluid-filled spaces (Hendricks et al., 1988). The remaining brain has been termed the “anencephalic area cerebrovasculosa.” In exencephalic brain tissue, the gyri and sulci are shallow, flattened, and disorganized. All surfaces of the brain are highly vascular. The remaining central nervous system tissue is dysplastic, with little or no neuronal differentiation, and very little normal cortex (Hendricks et al., 1988).